Today was the main event at the Supreme Court, debating the constitutionality of the individual mandate. The Court’s audience, including more than a few members of Congress, was full for the marathon two-hour argument. Even near the back of the courtroom, I shared a bench with three senators — the Court’s VIP section was obviously inundated.

Solicitor General Verrilli had a rough start to his argument, speaking haltingly, stumbling, and stopping to take a drink. The solicitor general spent almost all his time trying to convince the justices that health care is, in fact, different from other markets. While Justices Ginsburg and Kagan were trying to throw him soft balls, Verrilli kept striking out with Justices Scalia, Roberts, and Alito, and to some extent, Kennedy.

Justice Kennedy was particularly concerned because, as he put it, the government bears a “heavy burden of justification” when a law “changes the relationship of the individual to government in a unique way.” From my reading, General Verrilli didn’t ultimately convince them, and Justice Kennedy returned to the issue several times. He asked whether the administration’s argument had any limits “at all,” and noted that the mandate “requires the individual to do an affirmative act,” a completely novel type of law.

The Chief Justice and Justice Scalia were most vocal on this issue, the Chief declaring that “all bets are off” if they accept the expansive interpretation advanced by the administration.

Justice Kennedy probed Paul Clement, who was arguing for the 26 state respondents, on whether Americans “are in the market in the sense that they create a risk the market has to account for.” Justices Kagan and Breyer were firmly in this camp, with Breyer content that the risks alone were enough to bring people into a market. He went so far as to suggest that being born was equivalent to entering the health-care market. Justice Kagan was in favor of not “slicing the bologna too thin” by paying attention to the details of whether and when people enter a market.

An analysis by Judge Sutton of the Sixth Circuit got some play as Justice Kagan signaled twice that she might view the challenge differently if it were an as-applied challenge. For example, she suggested a Christian Scientist who objects to heath care might make a better plaintiff.

Justice Ginsburg, unsurprisingly, sent clear signals that she accepted the administration’s position. While she agreed that the mandate is a form of cross-subsidization, she was untroubled by this, given the cost-shifting that happens due to uncompensated health care.

Justice Breyer was perfectly comfortable with Congress creating commerce ex nihilo, a position even the solicitor general went to great (and, I believe, illogical) lengths to distance himself from. The solicitor general insisted on making the demonstrably false statement that the mandate in fact is not “creating commerce,” but is regulating actual market participants.

General Verrilli justified rounding the number of participants in the health-care market up to 100 percent simply because over 80 percent of Americans use health care yearly. But even a small percentage of the American population still represents millions of individuals.

Justice Kennedy expressed concern that the “uniqueness of the health-care market” would not operate as an effective limit on the Commerce Clause, because Congress would just say something else is unique next year. He nonetheless queried whether the line between participants and non-participants was not “uniquely proximately very close.” If Justice Kennedy does decide to uphold the individual mandate, he will try to construct a limiting principle, and hopefully one that is more defined than the obviously unadministrable question of “unique, proximate, very-closeness.” This is why people speculate the Chief Justice may vote with Justice Kennedy if he decides to uphold the law, in order to provide a more workable limiting principle.

Today’s argument indicates that those predicting a lopsided decision in favor of the mandate should start getting used to disappointment, and those who value their constitutional protections have good reason for optimism.